A subsea production system comprises a wellhead, subsea production tree equipment, pipelines, structures, and a piping system, etc., and, in many instances, a number of wellheads are controlled from a single location. A subsea control system is part of a subsea production system. The control system provides operation of valves and chokes on subsea completions, templates, manifolds and pipelines. Proper performance of the control system ensures reliable and safe operation of the production equipment.
The control of various production functions, executed at the sea bed, is carried out from a topside production facility (a platform or a floating vessel). The selection of the type of control system is dictated predominantly by technical factors like the distance between control points (offset distance between the platform and the tree), water depth, required speed of response during execution of subsea functions and type of subsea installation (single or multiple wellheads).
Topside control system equipment comprises a hydraulic power unit (HPU), an electronic power unit (EPU), and a well control panel. The HPU provides high and low-pressure hydraulic supplies and is usually powered by electric motors. The topside control system also comprises a Master Control Station (MCS). The MCS is a dedicated system that controls and retrieves data from subsea equipment on the ocean floor through subsea control modules (SCM) connected with subsea equipment, such as valves on the subsea production tree. Typically, a power and electronic control signal umbilical connects the MCS and SCMs but the connection may be wireless as well. The electrical control cables supplying power and control signals can either be bundled with hydraulic lines or laid separately.
The production control system provides control of all functions of the subsea production system. The production control systems, as such, are only concerned with controlling production and safety valves and monitoring devices and are not used to provide control of subsea connector latching and unlatching or operation of vertical access valves, for example. Typically, subsea functions include operation/control of: (1) a downhole safety valve (DHSV); (2) subsea chokes; (3) production valves mounted on the subsea production tree; and (4) utility functions such as monitoring of fluid characteristics, pressure leakage and valve positions, etc.
Positioned between the distributed control system (DCS) on the platform and the subsea equipment, the MCS maintains safe operating conditions, optimizing production across a field and effectively managing reserves.
The MCS includes electronic databases that serve as the communication link to the Distributed Control System (DCS), Hydraulic Power Unit (HPU), and Electrical Power Unit (EPU).
The MCS typically has three main layers: (1) a user interface, (2) logic/control, and (3) subsea communications to control components on the ocean floor. The MCS may use two complete and segregated channel networks to simultaneously monitor data functions to and from each other, as well as between surface and subsea equipment. The MCS communicates with the SCMs through subsea gateways that each consist of an SCM communication application and a database client. The logic component typically comprises a Programmable Logic Controller (PLC) system. However, PLCs can be limited in their expansion capabilities, ease of use and maintainability, remote monitoring, condition monitoring, and interfacing with third party equipment.
Traditionally for offshore production systems, the MCS is a redundant system that controls the subsea production system from one location, such as offshore, or a second location, such as onshore.